The use of membranes for fluid separation is becoming increasingly more common. In these systems, a fluid mixture under relatively high pressure is passed across the surface of a membrane adapted to act as a selective barrier, permitting some components of the fluid composition to pass through more readily than others. The separation of gases in membrane separation processes is generally due to molecular interaction between the gaseous components of the feed stream and the membrane. Because different components interact differently with the membrane, their permeation rates through the membrane are different, and substantial separation of components can be effected. While a selective effect can result from free molecular diffusion through membrane pores, especially in applications where small gas molecules such as hydrogen and helium are components of a gas mixture, membrane separation is often considered to proceed principally by the sorption of a gaseous component on the feed side of the membrane, diffusion of that component through the membrane, and desorption of the component from permeate side of the membrane. Membranes used for gas separation processes wherein the separation mechanism is controlled principally by solubility and diffusivity, as opposed to free molecular diffusion, are classified as nonporous membranes. While these nonporous membranes may in fact have small "pores", they are typically produced in a carefully regulated manner to provide a dense layer which effectively controls the gas transfer in the system. The structure of this dense control layer is often crucial to membrane performance, and it can be adversely affected by such factors as moisture, chemical degradation, or physical deformation.
Gas transfer through nonporous membranes is dependent upon variables such as membrane surface area, the pressure differential across the membrane, the diffusion rate of the gaseous components, and the effective thickness of the membrane. Generally, the membrane layer through which the gases must diffuse should be as thin as possible in order to obtain the maximum rate of gaseous diffusion. However, the membrane thinness is limited by a need to have a membrane free from defects, such as pinholes, and the need to have a membrane which has the physical integrity to withstand pressure differences sometimes as high as about 4,000 pounds per square inch (psi) across the membrane. For example, asymmetric cellulose ester membranes can be produced which do have a very thin but dense (nonporous) layer and a supporting sublayer of larger pore size. The thin dense layer basically controls the mass transfer in the system, and the thicker sublayer provides a degree of structural integrity. Many types of membranes, including cellulose esters and polymeric membranes, such as silicate rubber, polyethylene and polycarbonate, may be employed in gas separation. However, the particular membrane used can depend upon the separation sought to be effected.
Commercial gas separation processes are generally continuous operations in which a feed gas stream is brought into contact with the feed side of a membrane. The pressure on the feed side of the system is maintained at a pressure sufficiently higher than the pressure on the permeate side of the membrane to provide a driving force for the diffusion of the most permeable components of the gaseous mixture through the membrane. The partial pressure of the more permeable gaseous components is also maintained at a higher level on the feed side of the membrane than on the permeate side by constantly removing both the permeate stream and the residue of the feed stream from contact with the membrane. While the permeate stream can represent the desired product, in most gas permeation processes the desired product is the residue stream, and the permeate stream consists of contaminants which are removed from the feed stream.
For example, CO.sub.2 and H.sub.2 S can be removed from a hydrocarbon mixture, such as natural gas, using a thin dried supported cellulose ester membrane, and a differential pressure across the membrane of about 100 psi. The partial pressures of CO.sub.2 and H.sub.2 S in the permeate stream are preferably kept at about 80 percent or less of the partial pressure of those same components in the feed stream by separately and continuously removing the depleted feed gas (residue) stream and the permeate stream from contact with the membrane. The residue stream can, of course, be fed to another gas separation membrane stage to further reduce the concentration of CO.sub.2 and H.sub.2 S, and the permeate gas stream can likewise be fed to another separation stage to produce a product having a still higher concentration of the more permeable products CO.sub.2 and H.sub.2 S. In fact, the use of multiple separation steps in series and/or in parallel offers considerable diversity in separation alternatives using membrane technology so long as sufficient pressures can be maintained in the system.
Spiral wound membrane arrangements are commonly used in commercial fluid separation processes. An advantage of using a spiral wound technique is that this affords a large membrane contact area while permitting a rather small overall containment vessel. A standard way of supplying spiral wound membranes for commercial use is in the form of membrane elements which comprise a section of permeate conduit around which the membrane is wound. These membrane elements may then be used singly or joined together in series by interconnecting their permeate conduit sections. The usual way to use spiral wound membrane elements is to contain them, either singly or multiply in containment vessels to form fluid separation modules. The modules can then in turn be used singly or can be conveniently interconnected in series or parallel arrangements to provide the desired treatment.
When multiple membrane elements are used in series within a single module, it is desirable to seal each element from the other so as to inhibit the bypass of elements and the mixing of the respective feed fluids for the respective element. This is commonly accomplished by using a gasket or seal (e.g. U-cup seals) which seal the outer wrap of the element to the inner wall of the module containment vessel. The effectiveness of these seals depends on such factors as the type and condition of the seal material, and the surface of both the outer wrap of the element and the inner wall of the containment vessel. Thus membrane performance can be adversely affected by such factors as degraded seals and/or course-surfaced containment vessel walls. Moreover U-cup seals are typically designed to seal more firmly against the surfaces they address as the pressure drop across the seal increases. Inasmuch as the pressure drop across each membrane element is normally modest, there is some difficulty in providing U-cup seals which provide optimum sealing under the pressure drop conditions ordinarily encountered between two adjacent membrane elements in a series.